A motor-vehicle engine system may include a series of exhaust-aftertreatment devices that suppress feed gas emissions from an engine. These may include a nitrogen-oxide (NOX) trap, a diesel-type oxidation catalyst (DOC), a diesel-type particulate filter (DPF), and/or a selective catalytic reduction (SCR) device for reducing NOX.
Such devices may be arranged differently in different motor-vehicle exhaust systems. For example, U.S. Pat. No. 7,229,597 describes a diesel-engine exhaust system in which a DOC is arranged upstream of an SCR device. In U.S. 2007/0125072, a DOC is arranged downstream of an SCR device. Neither configuration may be suitable, however, for meeting increasingly strict emissions-control requirements with regard to NOX and non-methane hydrocarbons (NMHC). For example, the partial-zero-emission vehicle (PZEV) standard for light-duty diesel trucks tolerates no more than 0.02 grams NOX per mile and 0.01 grams NMHC per mile at 150,000 miles. A single DOC arranged in a diesel-engine exhaust system may have difficulty keeping NMHC below these levels, due mainly to excessive transmission of NMHC during cold-start conditions.
The inventors herein have recognized this issue and now disclose a series of approaches to address it. One embodiment provides a motor-vehicle engine system comprising a first DOC configured to receive exhaust from an engine, and an SCR device coupled downstream of the first DOC in a flow direction of the exhaust. The SCR device is configured to sorb a hydrocarbon at a lower temperature and release the hydrocarbon at a higher temperature. The system further comprises a second DOC coupled downstream of the SCR device. The second DOC is configured to oxidize the hydrocarbon. This novel configuration provides multiple distinct synergies. For example, much of the NMHC that slips past the first DOC may be temporarily stored in the SCR device until the second DOC has reached its light off temperature. By the time the NMHC is released from the SCR device, the second DOC will be sufficiently heated to provide effective NMHC control.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted herein.